The invention relates to a method for making a semiconductor device having a pattern of highly doped regions located some distance apart in a semiconductor substrate and regions of low doping located between the highly doped regions, wherein
a doping material is applied to the substrate, at least in the location of the highly doped regions,
the substrate is subjected to a diffusion step in which atoms diffuse from the doping material into the substrate, and
conducting contacts are made above the highly doped regions.
A method for making a selective emitter in a p-type crystalline Si substrate, with which a diffusion material in the form of a doping paste, such as phosphorus paste, is applied to the substrate by screen printing is described in J. Horzel, J. Szlufeik, J. Nijs and R. Mertens, xe2x80x9cA simple processing sequence for selective emittersxe2x80x9d, 26th PVSC, September 30-October 3; Anaheim, Calif.; 1997 IEEE pp 139-142. The substrate is then dried on a conveyor belt and placed in a diffusion furnace. During the diffusion step the doping materials diffuse into the substrate while diffusion material moves to the regions outside the imprint of doping material via the gas atmosphere in the furnace. Relatively deep diffusion zones having a phosphorus concentration varying from 1020 at the surface of the substrate to 1017 at a depth of 0.5 xcexcm below the substrate surface are formed below the imprinted dope material. Shallow diffusion zones having a low phosphorus concentration, varying from 1019 at the substrate surface to 1018 at a depth of 0.2 xcexcm, are formed outside the region of the imprint.
The disadvantage of the known method, in particular in the case of the production of solar cells in which the highly doped regions are arranged in a pattern of a series of parallel tracks or fingers, is that the diffusion between the tracks having a high concentration is highly sensitive to the atmosphere in the diffusion furnace, as a result of which the diffusion method is insufficiently stable as a production process. Furthermore the ratio between the high and low doping is dependent and therefore local doping cannot be adjusted to the optimum. To obtain good contact with the metalization placed on the highly doped regions, which metalization is frequently applied by screen printing, a low surface resistance, and thus as high as possible doping, is desired. For the regions located between the metalization an increase in yield is possible, for example in the case of n-p-type solar cells, by passivation of the surface with thermal SiO2 or PECVD SiN, as a result of which recombination of charge carriers at the surface is counteracted. This increase in yield can be achieved only if the doping is low.
One aim of the present invention is therefore to provide a method for making a semiconductor device, in particular a solar cell, with which regions of high and low doping can be applied efficiently in accurately determined positions on the substrate. A further aim of the invention is to provide a method with which the concentrations of the doping material in the regions of high and low doping can be adjusted relatively independently of one another.
To this end the method according to the invention is characterized in that before the diffusion step a diffusion barrier material is applied to the substrate at the location of the regions of low doping by imprinting with the barrier material in the pattern of the regions of low doping.
During the diffusion step, which usually will be carried out at temperatures of approximately 900xc2x0 C., the substrate regions located beneath the barrier material are shielded by the latter from the diffusion material applied to the neighboring regions. As a result the concentration in the regions of low doping can be freely adjusted accurately and independently of the concentration in the highly doped regions. Furthermore, with the method according to the invention a single screen printing step and a single drying step can suffice.
It is possible first to apply the doping material to the substrate as a uniform layer, for example by spraying, and then to print the barrier material by means of a printing technique onto the regions of the substrate with low doping, after which the diffusion step is carried out. In this embodiment the barrier material can delay the diffusion of the underlying diffusion material or it can have etching properties, so that the underlying diffusion during the diffusion step is etched out of the substrate. A barrier material which has etching properties is, for example, ZnO.
Alternatively, according to the invention the barrier material is first applied by screen printing, stencil printing, offset printing or tampon printing or using other printing techniques known per se to those regions of the substrate which are to have low doping. The doping material can then be applied as a single layer by spraying, spinning, immersing, vapor deposition or from the gas phase (such as, for example, by means of POCl3 gas in a crystal tube) on top of the substrate and on top of the barrier material.
Although this is not to be preferred from the production standpoint, the doping material can also be printed selectively onto the regions of the substrate for high doping, before or after applying the barrier material. The barrier material is, for example, a dielectric material such as Si3N4, SiO2 or TiO2, to which an n-type doping material, such as phosphorus (P), arsenic (As), antimony (Sb) or bismuth (Bi) can have been added, or a p-type doping material such as boron (B), aluminum (Al), gallium (Ga), indium (In) or thallium (Th). This material is printed onto the substrate in paste form and then sintered at temperatures between 200xc2x0 C. and 1000xc2x0 C.
Following the diffusion step the surface resistance in the highly doped regions is for example, between 10 and 60 ohm square, for a concentration of doping atoms of between 1018 cmxe2x88x923 and 1021 cmxe2x88x923, for a diffusion depth beneath the substrate surface of between 0.1 xcexcm and 0.5 xcexcm. The surface resistance of the regions with low doping is between 40 ohm and 600 ohm square, for a concentration of doping atoms of between 1017 cmxe2x88x923 and 1021 cmxe2x88x923, for diffusion depth of between 0.1 xcexcm and 0.5 xcexcm.